Fermented alcohols from carbohydrates such as sweet potatoes, corns, etc. are important raw materials for drinking or industrial use, but an aqueous alcohol solution obtained by the fermentation method has a low alcohol concentration, i.e. 10 to 20% by weight and accordingly, it is required to concentrate it to about 95 to 100% by weight.
Up to the present time, a distillation method has been employed as the concentrating method, but this distillation method is not economical because of difficulty in recovering the evaporation latent heat of alcohol and water, as a predominant component. Thus, it has eagerly been desired to develop a concentrating method of the energy saving type.
In the conventional distillation method, an energy level corresponding to about 3000 kcal/kg.multidot.alcohol is required for concentrating alcohol from 10% by weight to 95% by weight. For concentrating alcohol from 95% by weight to absolute alcohol of at least 99.2% by weight, azeotropic distillation using diethyl ether, benzene or cyclohexane has been carried out with about 1000 to 2000 kcal/kg.multidot.alcohol. Thus, energy saving is also desired.
On the other hand, there has been proposed a method comprising extracting and separating alcohol from water by the use of carbon dioxide under supercritical state or pseudocritical state, thereby concentrating the alcohol, as a concentrating method of the energy saving type (Japanese Patent Laid-Open Publication Nos. 56201/1981 and 141528/1984).
In the case of using carbon dioxide as a solvent, however, it has lately been reported that the solubility of alcohol is so low that a large amount of carbon dioxide is required, the selective extraction of alcohol is limited and the maximum alcohol concentration is limited to about 91% by weight. According it is impossible to obtain a concentration exceeding this limit.